The present invention relates to a process for producing hard carbonaceous sheets and, more particularly, to a process for readily producing hard carbonaceous sheets having small porosity, high mechanical strength, high elastic modulus, high hardness, isotropy and uniform thickness of several ten microns to several millimeters.
Such carbonaceous sheets can be applied to packings, gaskets, or can reduce the weight of an electrode or a shielding sheet for a chemical plant. These carbon sheets have been trially produced by a variety of processes, but thin carbon sheets which satisfy the above-described conditions cannot yet be obtained. For instance, non-impregnation graphite is produced by ordinarily impregnating a carbon material with a phenol resin or furan resin and then curing the impregnated carbon material, or by impregnating, curing and then recalcining, or repeatedly impregnating, curing and then calcining. However, the porosity of the carbon material thus produced is small (K =10.sup.- 3 cm.sup.2 /s), has mechanical strength twice as that of a base material with excellent corrosion resistance, but the manufacturing processes are not only complicated, but only thick products can be fabricated. The reduction in the weight of the product is almost impossible. More specifically, the carbon material is ordinarily produced in a block shape. When thin sheets are produced, the carbon material of this block shape should be cut. However, it was extremely difficult to cut the carbon material of the block shape into thin sheets having a thickness of 1 mm or less, and the carbon material can be cut into thin sheets having at least several mm at the thinnest. Even if the carbon material of the block shape can be cut, it is difficult to cut the material into thin sheets after impregnation of resin so as not to cause cracks or crazes. Even if the carbon material is cut into thin sheets without impregnation, the hardness of the carbon material is remarkably increased. As a result, an expensive cutter should be employed. In addition, a cutting technique of high precision is necessary, with the result that, even if such thin sheets can be produced, its cost extremely increases. An example of succeeding the reduction in the thickness of the carbon product is a sheet-like flexible graphite. Such sheet-like graphite was produced by acid treating natural graphite, heating the graphite to expand the graphite, and the graphite is then rolled and shaped into sheets having a thickness of several microns. In addition, its porosity is small to 2 x 10.sup.4 cm/s. However , such sheets have bending strength nearly equal to zero, and cannot accordingly be applied to a product, to which bending stress is applied. Since a large quantity of strong acid is used in the manufacturing processes, corrosion resistance and drainage of the apparatus for performing the process must be sufficiently paid, thereby causing high cost. On the other hand, the carbon material has non gas permeability in the same degree as glass (K=10.sup.-10 to 10.sup.-12 cm.sup.2 /s), extremely high mechanical strength, isotropy and extremely small surface area as vitreous carbon. This vitreous carbon is obtained by employing as base material thermosetting synthetic resin such as furan resin, phenol resin, adding a suitable hardening agent, hardening the resultant mixture at room temperature or slightly higher than room temperature for a long period of time such as 1 to 6 weeks, and calcining the carbon at a slow temperature rising velocity so as to prevent cracks due to the projection of volatile contents produced by thermal decomposition. In this manner, the structure of the conventional vitreous carbon not only requires calcining for a long period of time but it is difficult to obtain thin sheets from the observation of the manufacturing processes. In addition, the vitreous carbon has extremely high hardness, and the vitreous carbon of the block shape cannot be cut into thin sheets more difficult than the case of non-impregnated graphite.